1. Field of the Invention
The present invention relates to a laser irradiation method and a laser irradiation apparatus for using the method (apparatus including a laser and an optical system for guiding laser light emitted from the laser to an object to be irradiated). In addition, the present invention relates to a method of manufacturing a semiconductor device, which includes a laser light irradiation step. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light emitting device and an electronic device which includes the electro-optical device as a part.
2. Description of the Related Art
In recent years, a wide study has been made on a technique in which laser annealing is performed for a semiconductor film formed on an insulating substrate made of glass or the like, to crystallize the film, to improve its crystallinity so that a crystalline semiconductor film is obtained, or to activate an impurity element. Note that a crystalline semiconductor film in this specification indicates a semiconductor film in which a crystallized region is present, and also includes a semiconductor film which is crystallized as a whole.
A method of forming pulse laser light from an excimer laser or the like by an optical system such that it becomes a square spot of several cm or a linear shape of 100 mm or more in length on a surface to be irradiated, and scanning the laser light (or relatively shifting an irradiation position of the laser light with respect to the surface to be irradiated) to conduct annealing is superior in mass productivity and is excellent in technology. The “linear shape” described here means not a “line” in the strict sense but a rectangle (or a prolate ellipsoid shape) having a high aspect ratio. For example, it indicates a shape having an aspect ratio of 10 or more (preferably, 100 to 10000). Note that the linear shape is used to obtain an energy density required for sufficiently annealing an object to be irradiated. Thus, if sufficient annealing is conducted for the object to be irradiated, it may be a rectangular shape or a sheet shape. Under the present conditions, an excimer laser of 15 J/pulse is on the market. In the future, there is also a possibility that annealing with sheet shaped laser light is conducted.
FIGS. 7A and 7B show an example of a configuration of an optical system for forming laser light in a linear shape on a surface to be irradiated. This configuration is extremely general. All optical systems described above are based on the configuration shown in FIGS. 7A and 7B. According to the configuration, a cross sectional shape of laser light is converted into a linear shape, and simultaneously an energy density distribution of laser light on the surface to be irradiated is homogenized. In general, an optical system for homogenizing the energy density distribution of laser light is called a beam homogenizer.
Laser light emitted from a laser 101 is divided in a direction perpendicular to a traveling direction thereof by a cylindrical lens group (hereinafter referred to as a cylindrical lens array) 103, thereby determining a length of linear laser light in a longitudinal direction. The direction is called a first direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the first direction is changed in accordance with a direction of light bent by the mirror. In the configuration shown in the top view of FIG. 7A, the cylindrical lens array is divided into seven parts. Then, the laser lights are synthesized on a surface to be irradiated 109 by a cylindrical lens 105, thereby homogenizing an energy density distribution of the linear laser light in the longitudinal direction.
Next, the configuration shown in the cross sectional view of FIG. 7B will be described. Laser light emitted from a laser 101 is divided in a direction perpendicular to a traveling direction thereof and the first direction by cylindrical lens arrays 102a and 102b, thereby determining a length of linear laser light in a width direction. The direction is called a second direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the second direction is changed in accordance with a direction of light bent by the mirror. In the cross sectional view of FIG. 7B, the cylindrical lens arrays 102a and 102b each are divided into four parts. The divided laser lights are temporarily synthesized by a cylindrical lens 104. After that, the laser lights are reflected by a mirror 107 and then condensed by a doublet cylindrical lens 108 so that they become again single laser light on the surface to be irradiated 109. The doublet cylindrical lens 108 is a lens composed of two cylindrical lenses. Thus, an energy density distribution of the linear laser light in a width direction is homogenized.
For example, an excimer laser in which a size in a laser window is 10 mm×30 mm (which each are a half-width in beam profile) is used as the laser 101 and laser light is produced by the optical system having the configuration shown in FIGS. 7A and 7B. Then, linear laser light which has a uniform energy density distribution and a size of 125 mm×0.4 mm can be obtained on the surface to be irradiated 109.
At this time, when, for example, quartz is used for all base materials of the optical system, high transmittance is obtained. Note that coating is preferably conducted for the optical system such that transmittance of 99% or more is obtained at a frequency of the used excimer laser.
Then, the linear laser light formed by the above configuration is irradiated with an overlap state while being gradually shifted in a width direction thereof. Thus, when laser annealing is performed for the entire surface of an amorphous semiconductor film, the amorphous semiconductor film can be crystallized, crystallinity can be improved to obtain a crystalline semiconductor film, or an impurity element can be activated.
Also, an area of a substrate used for manufacturing a semiconductor device is being increased more and more. This is because high throughput and a low cost can be realized in the case where a plurality of semiconductor devices such as liquid crystal display device panels are manufactured from a single large area substrate as compared with, for example, the case where TFTs for a pixel portion and driver circuits (source driver portion and gate driver portion) are formed on a single glass substrate, thereby manufacturing a single semiconductor device such as a liquid crystal display device panel (FIG. 9). At the present time, for example, a substrate of 600 mm×720 mm, a circular substrate of 12 inches (about 300 mm in diameter), etc. are used as the large area substrate. Further, it is expected that a substrate in which a length of one side exceeds 1000 mm will be also used in future.
In end portions of linear, rectangular shaped, or sheet shaped laser light produced on the surface to be irradiated or its vicinity by the optical system, an energy density is gradually attenuated by an aberration of a lens or the like (FIG. 8A). In this specification, regions in which an energy density is gradually attenuated in end portions of linear, rectangular shaped, or sheet shaped laser light is called attenuation regions.
Also, with increase in an area of a substrate and an output of a laser, longer linear laser light, longer rectangular-shaped laser light, and larger sheet-shaped laser light are being produced. This is because high efficiency is obtained in the case where annealing using such laser light is conducted. However, an energy density in end portions of laser light emitted from an oscillating laser is lower than that in a substantially central region thereof. Thus, when an area of the laser light is expanded to be equal to or larger than an area up to now by the optical system, the attenuation regions tend to be increasingly noticeable.
In the attenuation regions of laser light, the energy density is insufficient as compared with a region having high homogeneity of an energy density and is gradually attenuated. Thus, when annealing is conducted using laser light having the attenuation regions, uniform annealing cannot be conducted for an object to be irradiated (FIG. 8B). In addition, even when annealing is conducted by a method of performing scanning with attenuation region overlapping of the laser light, the annealing condition is distinctly different from that for the region having the high homogeneity of the energy density. Thus, uniform annealing cannot be still conducted for the object to be irradiated. Therefore, the same treatment cannot be conducted for a region of the object annealed by the attenuation regions of the laser light and another region of the object annealed by the region of the laser light having the high homogeneity of the energy density.
For example, when the object to be irradiated is a semiconductor film, crystallinity of a region of the film annealed by the attenuation regions of the laser light is different from that of another region of the film annealed by the region of the laser light having the high homogeneity of the energy density. Thus, even when TFTs are manufactured from such a semiconductor film, electrical characteristics of TFTs manufactured from the region of the film annealed by the attenuation regions of the laser light are deteriorated and this becomes a factor for causing a variation of TFTs on the same substrate. Actually, there is almost no such a case where the TFTs are manufactured from the region of the film annealed by the attenuation regions of the laser light to produce a semiconductor device. Thus, this becomes a factor for decreasing the number of usable TFTs per substrate, thereby reducing throughput.